1993 Opel Astra CD (2 Litre)

[PBG]

I have a 1993 Astra which has started cutting out when idling, and sometimes cutting out while under load but revs below 1000 rpm.
When the engine is cold, it gradually loses power for a time (maybe 10 seconds) before returning to normal, almost as though the engine was starved of air or petrol.
I've had it tuned and cleaned all around the throttle body/inlets. The injectors have been cleaned, the fuel filter changed, and the fuel pump cleaned.
When the engine is running, the orange engine warning light frequently illuminates, sometimes for a very brief time, sometimes for the rest of the journey (usually no more than 20 minutes). I have noticed (but only once or twice) that the engine doesn't cut out when the engine warning light is illuminated, and I don't think (but aren't sure yet) it loses power when cold if the engine warning light is illuminated.
Any ideas?